The lagoon Kamo Lake is located in the north-eastern part of Kuni-naka Plain, Sado Island, Japan. The lagoon is separated from Ryotsu Bay by a sand bar where Ryotsu Town is situated. It is filled with marine water except for the thin surface parts, because there is a narrow waterway which was made artificially between the lagoon and sea in the northern site. The water and sediment of Lake Kamo were examined on August 28-30, 1996 and August 22 - 24, 1997. The lagoon is divided into three areas, outer, central and inner areas. The lagoon water was divided into two or three different layers based on water temperature, salinity and so on. Salinity values of the lake water is nearly equal to the sea water except near river mouth and in the surface layer during heavy rain. Generally, there is a jumping layer of salinity about 3m in depth. The salinity of the deep layer is higher than in the surface layer. In 1996, the temperature of the surface layer water was a little lower than the middle layer water where an inversion layer of water temperature was formed. The temperature of the deep layer water gradually decreased downward. In 1997, the inversion layer was not seen in the middle layer. In 1996, supersaturated water-soluble oxygen was distributed over wide areas and water-soluble oxygen was detected just above the bottom. On the other hand, there was no or less water-soluble oxygen layer in 1997. The transparency of lake water is 4 - 5m in general and decreases toward the coastal line. The lagoon sediments are mainly composed of gray or black mud except for the marginal part of the lake where gravels and sands are distributed. The muddy sediments of central and inner areas everywhere are black-colored with sulfuretted hydrogen smell. Many kinds of marine planktonic and benthic biota were found in the lagoon in 1996.